1. Field of the Invention
This application relates generally to oscillators and specifically to adjusting the frequency of an oscillator circuit in response to changes in the power supply voltage.
2. Description of Related Art
Ring oscillator circuits are well known and may be used in a variety of circuit implementations. For example, charge pumps may employ a ring oscillator circuit to provide a clocking signal according to which the various stages of the charge pump are successively charged and discharged. The frequency of the clocking signal generated by the ring oscillator circuit typically varies over the operating voltage range of the power supply. As the power supply voltage decreases below approximately 3.0 volts, the frequency of the clocking signal generated by the ring oscillator circuit may, during such fluctuations described above, decrease to a level which degrades the performance of an associated circuit. For instance, when used as mentioned above with a charge pump circuit, the frequency of the clocking signal provided to the charge pump circuit may decrease to a level which undesirably limits the maximum current delivered to the charge pump, thereby retarding the charging of the charge pump.
The above-described problem of frequency variations has led to the development of ring oscillator circuits which generate a clocking signal having a frequency substantially independent of fluctuations in the magnitude of the power supply voltage. However, as the voltage of the associated power supply decreases, more current is required to sustain a constant charging of the charge pump circuit. Thus, there is a need to provide to charge pump circuit a clocking signal whose frequency increases with decreases in the voltage of the power supply.